The present invention relates to a method and apparatus for use in dilating occluded blood vessels. The invention is particularly concerned with such a method and apparatus wherein dilatation is achieved through means of a balloon element of large diameter which is inflated to compress the occlusion being treated and wherein the transverse dimension of the non-inflated balloon element may be reduced to a minimal condition to enable placement of the catheter properly within the lumen of an occluded artery or vein.